I posted the site for a main gurdle, because someone suggested it. I too have 2-400 blks, a 2 bolt main 377" on alcohol, that has been welded 3/4" below the deck on one side about 4" lg. it's filled to the bottom water pump bolts. It's in the car right now, i don't know how much HP, but it runs mid 6's 1/8 and mid 10's in the 1/4 @3250lbs. I also have a 4 bolt that ran the same #'s at 406" on gas, in the same car no filler. I donot have a gurdle on either of these engine, and the 2 bolt has hundreds of passes on it, the 4 bolt broke a rod and shortened it's life, but the blk was still o'k and i'm rebuilding it for the street. I've probably ran 10 of these blk's in the last 20 yrs. no gurdle, with pretty good success.
JMO
Zip.